1. Field of the Invention
The present invention generally relates to a MEMS (Micro Electro Mechanical System) for RF (Radio Frequency). More particularly, the present invention relates to a MEMS switch for RF that can be driven at a low voltage and a method for manufacturing the same.
2. Description of the Related Art
Generally, MEMS is a micro electro mechanical system that is manufactured using a semiconductor process. Recently, MEMS has been the focus of increased attention as a range of applications of MEMS technology has increased in connection with the development of mobile communication technology. Among such MEMS applications, a gyroscope, an acceleration sensor, an RF switch, and the like are being applied to products. In addition, the development of various other MEMS products has accelerated.
A MEMS RF switch is embodied to switch a signal when a micro-sized MEMS structure on a semiconductor substrate moves to contact a signal electrode or to intercept a signal transmission when the MEMS structure is separated from the signal electrode. This MEMS switch has advantages in that it exhibits a lower insertion loss upon being switched “ON” and a higher attenuation coefficient upon being switched “OFF,” as compared to conventional semiconductor switches. Further, it requires a significantly lower switch driving power than conventional semiconductor switches. In addition, it has gained public attention as a device suitable for RF communication since an application frequency range thereof extends up to about 70 GHz.
However, this MEMS switch for RF also has a problem in that it requires a high driving voltage since it uses an electrostatic force and a stiction, i.e., static friction, phenomenon may occur at a contact point. The stiction phenomenon describes an unintended and undesirable adhesion that occurs on a surface of a microstructure when a restitution force does not overcome interfacial forces, such as a capillary force, a van der Waals force, an electrostatic attraction, and the like, thus causing the contact point to become stuck, either permanently or for an unwanted period of time. The stiction phenomenon may be generally classified into two types, a sacrificial layer release-related stiction and an in-use stiction. The first type, the sacrificial layer release-related stiction, is an adhesion referring to a circumstance, in which a structure sticks at a bottom and is not released therefrom because of a liquid capillary force during an intended release of the structure. This phenomenon may be solved by technologies to avoid a liquid-vapor interface, such as sublimation release, supercritical drying, HF vapor release, and the like. In addition, there is a method to reduce the capillary force by forming a small protrusion around the microstructure to change a liquid meniscus.
However, these methods cannot additionally avoid occurrence of the second type, the in-use stiction, in which a microstructure is not restored due to humidity or an excessive impact generated while in use. This occurs because when surfaces of adjacent microstructures contact each other, a capillary force, an electrostatic attraction, a van der Waals force, or the like, are also generated and surface adhesion may occur due to these forces, whereby stiction of the structure takes place, causing damage to a device. In an attempt to solve the in-use stiction, a method to reduce a surface contact area by forming a micro dimple and a method to manufacture a polycrystalline silicon surface to a microscopic level have been proposed. In addition, methods to modify a surface of a microstructure using chemicals, i.e., chemical modification of the surface, have been proposed. The proposed chemical modification methods include use of hydrogen passivation, hydrogen-bonded fluorinated monolayers, plasma-deposited fluorocarbon thin films, covalently-bound hydrocarbon self-assembled monolayer (SAM), and others. Among these, a representative method is the self-assembled monolayer (SAM) method. The SAM method is a technology to prevent the stiction phenomenon by subjecting a silicon wafer surface to a chemical. However, the SAM method has several disadvantages, e.g., requiring complex treatment procedures, a significant cost of production, and a high dependency on temperature.
As described above, with respect to a MEMS switch for RF, research has been conducted on all aspects of the device to solve problems related to stiction, however, there is still a demand for a more economical and effective embodiment to be applied in industrial products. Therefore, attempts have been made to apply MEMS structures and driving methods thereof as a low-cost solution for the stiction phenomenon.
FIGS. 1A and 1B illustrate a plan view and a cross-sectional view taken along line 2–2′ of FIG. 1A, respectively, of a conventional MEMS switch for RF. Referring to FIGS. 1A and 1B, the conventional MEMS switch for RF includes a driving electrode 16 formed on a substrate 12, a transmission line 18 having a cut region, a cantilever beam support pillar 14, a cantilever beam 20 formed a particular distance from the substrate, i.e., a predetermined height above the substrate, by means of the cantilever beam support pillar 14, an upper electrode 24 formed on the cantilever beam having a region facing a lower electrode 16, and a contact part 22 formed on a lower surface at an opposite end from the cantilever beam support pillar 14 of the cantilever beam 20 to face the cut region of the signal transmission line 18 to electrically connect the transmission line 18. Here, both the cantilever beam 20 and the upper electrode 24 have a spring part 23 connecting a region 26 facing the lower electrode 16 and an upper region of the support pillar 14 so that the cantilever beam can resiliently move upward and downward, as shown by arrow 11 in FIG. 1B. The spring part 23 connects the lower electrode facing region 26 and the upper region of the support pillar in a form of a narrow linear band.
In the above-described conventional MEMS switch for RF, a moving side of the cantilever beam 20, i.e., the side opposite from the side attached to the cantilever beam support pillar 14, can move downward by an electrostatic force generated due to a potential difference applied to the upper and lower electrodes 24 and 16 and this downward movement of the cantilever beam 20 allows the contact part 22 to electrically connect the cut region of the transmission line 18. Thus, a signal can pass along the transmission line 18. Alternatively, when the driving voltage applied to the upper electrode 24 and the lower electrode 16 is removed for signal interception, the contact part 22 is separated from the transmission line by a resilient resititution force of the cantilever beam 20 and returns to an original state. At this time, the spring part 23 helps the contact part to be separated more resiliently from the transmission line. That is, in an effort to solve problems related to stiction, the spring part is used to further increase the restitution force of the cantilever beam, as compared to a conventional cantilever beam without a spring part.
However, this conventional MEMS switch for RF has a problem in that the driving voltage necessary to move the cantilever beam 20 is increased. More specifically, the driving force F needed to move the cantilever beam 20 satisfies a relation directly proportional to the area A of the electrode but inversely proportional to the square of the distance d between the lower electrode 16 and the upper electrode 24 on the cantilever beam. However, when the spring stiffness of the cantilever beam 20 is raised to increase the restitution force needed to separate the contact part connected to the transmission line 18, additional driving force is needed to move the cantilever beam 20. In order to increase the driving force, the area of the electrode should be expanded or the driving voltage should be increased. Since an expansion of the area of the electrode may cause negative effects, such as an increase of adhesion, the driving voltage is raised to increase driving power. For this reason, conventional MEMS-type switches have a driving voltage exceeding 10 V. Consequently, such a high driving voltage of a MEMS switch for RF requires a separate circuit for increasing the voltage, which contributes to an increase in cost, since general portable terminals are normally driven at a voltage as low as 3 V.
In addition, the MEMS switches for RF having a bridge-type or a cantilever-type (cantilever beam-type) structure totally depend on stiffness of the structure when restituting the contact point. However, in a case like a switch, the time when the state conversion occurs is not regular and a duration of a state may be relatively long. Accordingly, when a state lasts for a relatively long period of time, creep (or memory effect) may occur, which inhibits restitution to the other state. That is, in a case of a bridge-type or cantilever-type MEMS switch for RF, since a state changing part always receives one type of stress, such as N-T-N (Neutral-Tension-Neutral) or N-C-N (Neutral-Compressive-Neutral), except during the initial state, it cannot be restituted to the original state when used for a long period of time, which causes deterioration in RF properties.